1. Field of the Invention
The present invention relates to a novel crosslinked polycarbonate and a process for producing the same. This crosslinked polycarbonate is useful as a modifier for polyesters, polyurethanes etc. and particularly useful as a modifier for polylactic acids important as biodegradable resins.
Further, the present invention relates to a polylactic acid type resin composition containing said crosslinked polycarbonate as a modifier. The polylactic acid type resin composition of the present invention is excellent in impact resistance, possesses practically adequate strength, flexibility and transparency and is utilized in packaging materials and a wide variety of molded articles.
The polylactic acid referred to in the present specification is intended to mean both homopolymers of lactic acid and copolymers of lactic acid.
2. Description of Related Art
As crosslinked polycarbonates, there are known polycarbonates produced by reacting diols and trihydric or more polyhydric alcohols such as trimethylolpropane, pentaerythritol etc. with carbonic acid diesters.
Japanese Laid-Open Patent Publication No. 3-220,233/1991 discloses crosslinked polycarbonates obtained by transesterification between polycarbonate diol and trimethylolpropane or pentaerythritol.
Japanese Laid-Open Patent Publication No. 6-73,173/1994 discloses crosslinked polycarbonates obtained by reacting a carbonyl component, a branched diol, and a tetrahydric to hexahydric or more polyhydric alcohol such as ditrimethylolpropane or sorbitol.
However, any trihydric or more polyhydric alcohols used as monomer components in the above-described conventional crosslinked polycarbonates contain 1,2-diol or 1,3-diol structural units. In a report of D. B. Pattison (J. Am. Chem. Soc., 79, 3455 (1957)), it is suggested that trimethylolpropane etc. with 1,3-diol structural units are subject to side reactions so that they are converted via cyclic carbonates into cyclic ethers by decarboxylation, so they are not necessarily suitable as the starting monomer component for crosslinked polycarbonates.
From a viewpoint of natural environmental protection in recent years, there is demand for biodegradable resin and molded articles thereof decomposed in the natural environment, and the research and development of biodegradable resins such as aliphatic polyesters etc. are actively conducted. In particular, polylactic acid type polymers not only have a melting point as considerably high as 170 to 180.degree. C. but are also excellent in transparency, so these are expected to be promising packaging materials or starting materials for molded articles making use of transparency. However, polylactic acids suffer from the drawback of poor impact resistance and brittleness due to their rigid molecular structure, so there is demand for improvements in these polylactic acid type polymers.
Generally, a polycarbonate resin made of bisphenol A as the starting material is transparent and superior in mechanical characteristics such as impact resistance and tensile strength, and there is increasing utilization thereof as materials directed mainly to industrial materials for general-purpose products. However, similar to other plastic products, this resin is not decomposed under the natural environment either, and the heat of combustion thereof upon thermal disposal is so great that a social problem arises as a cause of environmental destruction.
On the other hand, Japanese Laid-Open Patent Publication No. 8-187,090/1996 discloses aliphatic polyester carbonates as resins capable of biodegradation by microorganisms of the genus Pseudomonas. Further, Japanese Laid-Open Patent Publication Nos. 7-53,693/1995 and 7-53,695/1995 disclose that high-molecular-weight aliphatic polyester carbonates obtained from aliphatic dicarboxylic acids, aliphatic diols and diallyl carbonates are applicable to packaging materials and molded articles. However, the melting points of these aliphatic polyester carbonates are as low as 90 to 110.degree. C. although their biodegradability has been confirmed. In addition, aliphatic polyester carbonates are generally poor in mechanical strength including tensile strength, and even their highest tensile strength is as low as 40.9 MPa, as described in Japanese Laid-Open Patent Publication No. 8-134,198/1996, so their utilities are limited in cases where they are used solely in packaging materials or molded articles.
Japanese Laid-Open Patent Publication No. 8-27,362/1996 describes an example of the modification high-molecular-weight of polyester carbonates by fusion-mixing thereof with bacterially degradable poly-.beta.-hydroxybutyric acid (PHB) to utilize the rigidity of PHB. However, the tensile strength of even those with the highest improvement in rigidity is 32 MPa which is not sufficient as molded articles requiring strength. Further, because PHB is an opaque resin, it is highly estimated that the resulting molded article is also opaque.
Japanese Laid-Open Patent Publication No. 7-82,369/1995 describes a process for producing a bisphenol A type polycarbonate/lactide copolymer. According to this publication, the melting point of the resulting resin is as high as 124 to 176.degree. C. and its degradability in the natural environment has been confirmed. However, any of the resulting resin is whitened and opaque, thus limiting utilities thereof as packing materials.